Most of the components currently used in proton exchange membrane (PEM) fuel cells are derived from designs originally developed for use in phosphoric acid fuel cells (PAFC), and are not optimal for the higher performance of PEM fuel cells.
By the mid-70s, components consisting entirely of carbon were made for use in PAFC's operating at temperatures in the 165-185.degree. C. range. One particular manufacturer has made bipolar plates by molding a mixture of graphite powder (approximately 67 wt %) and phenolic resin (approximately 33 wt %) and carefully heat-treating to carbonize the resin without introducing excessive porosity by rapid degassing. Typically, heat treatment to 900.degree. C. was sufficient to give the required chemical, physical and mechanical properties. The bipolar plates were molded flat and were machined to produce the required fluid distribution or collection grooves (or cooling grooves for the bipolar plate). Somewhat later in time, grooved plates were molded in a die (which was slightly oversized to compensate for shrinkage during baking) to produce a glassy graphitic, carbon-composite plate. However, while carbon/graphite bipolar plates are effective, they are expensive and, because it is difficult to produce thin carbon based bipolar plates, stacks built with these plates tend to be heavy and bulky.
One alternative for overcoming these limitations is to use a moldable graphite-based composite that does not have to be carbonized. Graphite powder, which serves as the conductor, is bonded into a rigid piece with a polymer matrix. The graphite retains its conductivity and corrosion resistance, and the polymer binder, which must also be stable under PEM operating conditions, allows the plate to be formed by conventional polymer forming processes. This approach has distinct limitations. When the graphite is diluted with the polymer, its conductivity, already lower than any metal, is reduced even further. A seven kilowatt stack with pure graphite bipolar plates would be expected to have a 16 Watt internal resistive loss. When the graphite is dispersed in a polymer matrix, this loss will be larger.
Yet another example of a bipolar plate is a solid titanium metal sheet. The titanium is resistant to corrosion in many applications, provides greater electronic conductivity than does graphite, and can be made in relatively thin sheets. However, titanium is very expensive and relatively heavy itself.
Therefore, there is a need for a lightweight bipolar plate that provides the desired conductivity and can withstand the corrosive environments typically found in fuel cells and the like.